There are 2 objectives to this protocol: 1) to determine whether the gradual initiation of trimethoprim/sulfamethoxasole (TMP/SMX) therapy reduces the incidence of treatment-limiting adverse reactions compared to the routine initiation of the drug for PCP prophylaxis in HIV infected subjects, and 2) to determine whether the gradual initiation of TMP/SMX reduces the incidence of adverse reactions that are not treatment-limiting compared to the routine initiation of the drug.